Unsupported segments of subsea pipelines are subject to vibrations caused by shedding of vortexes when currents have a significant velocity component perpendicular to the pipeline axis. When spans of pipeline are unsupported for lengths of a few hundred feet, currents of a 100-year storm could cause vibrations due to vortex shedding that could lead to failure of a pipeline within a short time period. This problem has been recognized and has typically been dealt with by supporting the segment of pipeline at one or more points along the suspension, by trenching out the high areas of the sea floor that cause the segment to remain off bottom, or by adding weight to the segment.
The segments are typically supported by dumping rocks over the suspension, or weighted down by placing concrete mats over the pipeline. If the sea floor is soft enough to allow jetting or trenching, the soil supporting the pipe may be removed to allow the pipeline to lay on the sea floor. When the unsupported segments are in shallow water and are suspended only short distances above the sea floor, these methods are relatively inexpensive and effective. However, as production of oil and gas is expanded into deep waters past the edge of the continental shelf, installation of pipelines over rougher sea floor terrain is becoming common. Larger gaps between the sea floor and the unsupported segments of pipeline are caused by this rougher sea floor terrain. Adding weight to or supporting a pipeline are caused by this rougher sea floor terrain. Adding weight to or supporting a pipeline can be very expensive when a large gap between the sea floor and pipeline exists. The large gaps also expose the pipeline to greater current velocities due to the diminished boundary effect of the sea floor. Also, the depths, which these spans occur in, are beyond the safe diving range; hence, any work done on the span must be performed with remotely operated underwater vehicles or other specially designed machinery that does not require diver assistance. Reduction of vortex-induced vibrations by weighting down, trenching out, or supporting the pipeline can be very expensive when the pipeline is beyond the continental shelf.
Vortex-induced vibrations have been of concern in the design of off-shore tension leg platform tendons and risers. When supporting such segments is not practical, fairings are used to eliminate vortex shedding. Fairings create a streamlined flowpath, resulting in elimination of the vortex-induced vibrations around the segment even with relatively high velocity currents. Fairings are relatively expensive, and are difficult to install without the aid of divers.
It is therefore an object of the present invention to provide a method to reduce vortex-induced vibrations in an unsupported segment of a subsea pipeline wherein the segment is not tied or supported, and wherein the method includes placing a shroud around at least a portion of the segment. It is a further object to provide such a method wherein the shroud can be placed around the segment without the aid of divers. In another aspect of the present invention, it is an object to provide a shroud useful in such a method.